Laser device capable of a plurality of laser beams of different levels

ABSTRACT

In a solid-state laser device comprising a laser medium which is located within a plurality of resonance optical paths and which has a plurality of medium lengths forming parts of the resonance optical paths, at least one of the medium lengths within the resonance optical paths is selected such that a third level laser beam can be oscillated while at least one of the remaining medium lengths is different from the first of the medium lengths and is selected such that a fourth level laser beam is oscillated.

BACKGROUND OF THE INVENTION

This invention relates to a solid-state laser device for emitting aplurality of laser beams by the use of a single laser medium. Such aplurality of laser beams will be collectively called a multi-laser beam.

A laser device of the type described is known which is capable ofemitting a multi-laser beam by the use of a single laser medium. By wayof example, such a laser device is disclosed in a book which is writtenby Alexander A. Kaminskii and which is published 1981 by Spriger-Verlagunder the title of "LASER CRYSTALS", pages 111 to 113, Section 3.18Multibeam Crystal Laser. As shown in FIG. 3.52a of the above-mentionedbook, the laser device comprises the laser medium and four mirrorslocated around the laser medium. The laser medium is disposed in firstand second resonance optical paths which pass through the laser mediumwith first and second medium lengths, respectively. A first pair of themirrors is placed in the first resonance optical path with the lasermedium put therebetween while a second pair of the mirrors is placed inthe second resonance optical path with the laser medium puttherebetween. The first pair of the mirrors forms a first resonatorwhile the second pair of the mirrors forms a second resonator. Along thefirst resonance optical path, a first excitation laser beam is suppliedto the laser medium from a first excitation laser beam source to excitethe laser medium while a second excitation laser beam is supplied alongthe second resonance optical path to the laser medium from a secondexcitation laser source to excite the laser medium. Each of the firstand the second excitation laser beams may be supplied from semiconductorlaser elements.

The first resonator cooperates with the laser medium to pump the lasermedium and to generate a first output laser beam which is derived fromthe laser medium as one of the multi-laser beam. Similarly, the secondresonator cooperates with the laser medium to pump the laser medium andto generate a second output laser beam which is derived from the lasermedium as another multi-laser beam. In this event, the first outputlaser beam has a first wavelength which is equal to about 1.06micronmeters while the second output laser beam has a second wavelengthwhich is equal to about 1.35 micronmeters. In the laser device, each ofthe first and the second laser beams is excited by the four-level systemand may therefore be a fourth level laser beam. This is because each ofthe first and the second medium lengths is determined such that emissioncan be excited by the four-level system.

With this structure, no consideration is made at all about oscillationof a plurality of different level laser beams by the use of a singlelaser medium. Accordingly, it is impossible to generate a blue-coloredlaser beam which is very important for the production of three primarycolors.

SUMMARY OF THE INVENTION

It is an object of this invention to provide a solid-state laser devicewhich is capable of emitting a wide variety of laser beams which havedifferent wavelengths.

It is another object of this invention to provide a solid-state laserdevice of the type described, which can emit three primary colors oflight by the use of a single laser medium.

It is still another object of this invention to provide a solid-statelaser device of the type described, which is feasible for emitting ablue-colored laser beam.

A solid-state laser device to which this invention is applicable is foruse in emitting a plurality of laser beams. The solid-state laser devicecomprises a laser medium and a plurality of resonance optical pathswhich pass through the laser medium. The laser medium has a plurality ofinternal optical paths which form parts of the resonance optical paths.The internal optical paths include a first one of the internal opticalpaths that is selected such that a third level laser beam is generatedas at least one of the laser beams and a second one of the internaloptical paths that is selected such that a fourth level laser beam isgenerated as at least one of the remaining laser beams.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic view of a conventional solid-state laser device;

FIG. 2 is a schematic perspective view of a solid-state laser deviceaccording to a first embodiment of this invention; and

FIG. 3 is a similar view of a solid-state laser device according to asecond embodiment of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, a conventional solid-state laser device issubstantially equivalent to that described previously in the Backgroundof the Invention. The solid-state laser device comprises a laser medium11, a first resonator composed of a pair of reflection mirrors 12 and13, and a second resonator composed of another pair of reflectionmirrors 14 and 15. As illustrated in FIG. 1, the laser medium 11 has asquare shape having four sides which have an equal length L and may beformed by a neodymium-doped active material. The first and the secondresonators are tuned so that first and second laser beams 16 and 17 areemitted from the laser medium 11. Practically, the first and the secondlaser beams 16 and 17 have first and second emission wavelengths equalto 1.06 and 1.35 micronmeters, respectively. At any rate, the first andthe second laser beams are generated by exciting a fourth level by theuse of a four-level scheme.

With this structure, it is impossible to emit a laser beam from a leveldifferent from the fourth level. As long as the laser beams aregenerated by the use of the fourth level only, it is difficult to obtaina blue-colored laser beam having a wavelength of 0.47 micronmeter, aspointed out hereinbefore.

Referring to FIG. 2, a solid-state laser device according to a firstembodiment of this invention comprises a laser medium 21 formed as aslab-shaped rectangular parallelpiped which has upper and lowersurfaces, righthand and lefthand side surfaces, and front and rearsurfaces, as shown in FIG. 2. The upper and the lower surfaces may becalled first and second surfaces, respectively, while the righthand andthe lefthand side surfaces may be referred to as third and fourthsurfaces, respectively. In this connection, the front and the rearsurfaces may be called fifth and sixth surfaces, respectively. Eachsurface is subjected to mirror finishing and may therefore be consideredas mirror finished surfaces.

The laser medium 21 may be, for example, a neodymium-doped activematerial, namely, a matrix doped with a neodymium ion (Nd³⁺). In thisevent, the matrix may be, for example, a crystal, such as Al₂ O₃, YAG(Y₃ Al₅ O₁₂), YLF (LiYF₄), CaF₂, CaWO₄, or a glass material, such asLHG-5 or LHG-8 manufactured and sold by Hoya Corporation.

The laser medium 21 has a size specified by a width L1, a length L2, anda thickness L3, as shown in FIG. 2. The width L1, the length L2, and thethickness L3 are defined between the front and the rear surfaces of theillustrated laser medium 21, between the righthand and the lefthand sidesurfaces, and between the upper and the lower surfaces, respectively.Practically, the laser medium 21 is made of YAG and the thickness L3 is1.5 millimeters and the width L1 and the length L2 each is equal to 5millimeters.

It is to be noted that the front surface, the lefthand side surface, andthe lower surface are coated with first, second, and third reflectionfilms, respectively. Moreover, the rear surface is coated with a firstantireflection film for preventing reflection of a first laser beam 26which has a first wavelength of 1.06 micronmeters. In other words, thefirst antireflection film on the rear surface is transparent to thefirst wavelength of 1.06 micronmeters. On the other hand, the righthandside surface is coated with a second antireflection film for preventingreflection of a second laser beam 27 which has a second wavelength of1.32 micronmeters while the upper surface is coated with a thirdantireflection film for preventing reflection of a third laser beam 28which has a third wavelength of 0.94 micronmeter.

Herein, an A-axis, a B-axis, and a C-axis are defined which are normalto the front surface, the lefthand side surface, and the lower surface,respectively, and which are orthogonal to one another in the examplebeing illustrated. In this connection, it is readily understood that thewidth L1, the length L2, and the thickness L3 are measured along theA-axis, the B-axis, and the C-axis, respectively.

Further referring to FIG. 2, a first semi-transparent mirror 31 isdisposed so that the semi-transparent mirror 31 intersects the A-axisand confronts the front surface of the laser medium 21. The firstsemi-transparent mirror 31 cooperates with the first reflection filmcoated on the front surface of the laser medium 21 to form a firstresonator 31a. Likewise, a second semi-transparent mirror 32 is disposedalong the B-axis so that the second semi-transparent mirror 32intersects the B-axis and confronts the second reflection film coated onthe lefthand side surface of the laser medium 21 so as to form a secondresonator 32a between the second reflection film and the secondsemi-transparent mirror 32. In addition, a third semi-transparent mirror33 is disposed along the C-axis so that the third semi-transparentmirror 33 intersects the C-axis and confronts the third reflection filmcoated on the lower surface to form a third resonator 33a. First,second, and third optical paths 46, 47, and 48 are formed along theA-axis, the B-axis, and the C-axis within the first, the second, and thethird resonators 31a, 32a, and 33a, respectively. In this connection,first through third internal optical paths are formed within the lasermedium 21 along the first through the third optical paths 46 to 48,respectively.

A first semiconductor laser device 51, a second semiconductor laserdevice 52, and a third semiconductor laser device 53 are disposed alongthe A-axis, the B-axis, and the C-axis, respectively. The firstsemiconductor laser device 51 emits a first excitation laser beam 51'towards the front surface of the laser medium 21 coated with the firstreflection film. The first excitation laser beam 51' passes through thefirst reflection film into the laser medium 21 and pumps or activatesthe laser medium 21 through the front surface. This shows that the firstreflection film is transparent to the first excitation laser beam.

The second semiconductor laser device 52 emits a second excitation laserbeam 52' towards the lefthand side surface coated with the secondreflection film. Like the first excitation laser beam 51', the secondexcitation laser beam 52' passes through the second reflection film intothe laser medium 21 and pumps the laser medium 21 through the lefthandside surface.

Furthermore, the third semiconductor laser device 53 emits a thirdexcitation laser beam 53' towards the lower surface coated with thethird reflection film. As a result, the third excitation laser beam 53'passes through the lower surface into the laser medium 21 and pumps thelaser medium 21 through the lower surface. Each of the first through thethird excitation laser beams 51' to 53' may have the same wavelength.Practically, each of the first through the third semiconductor laserdevice 51 to 53 may have output power of 500 milliwatts and anoscillation wavelength of 0.807 micronmeter.

The first resonator 31a can be tuned to a fourth level laser beam whichhas a wavelength of 1.06 micronmeters and which may be called the firstlaser beam 26 as mentioned before. Specifically, the first reflectionfilm has a reflectivity of about 100% for the first laser beam and atransmittivity greater than 97% for the first excitation laser beamwhich may have a wavelength of 0.807 micronmeter. Such a firstreflection film can be obtained by stacking a plurality of dielectricfilms, as known in the art. The first semi-transparent mirror 31 has areflectivity of 97% for the first laser beam 26 having the wavelength of1.06 micronmeters.

The second resonator 32a is tuned to a fourth level laser beam which hasa wavelength of 1.32 micronmeters and which may be referred to as thesecond laser beam 27. To this end, the second reflection film is formedby a plurality of dielectric films which may be named a multipledielectric film and which have a reflectivity of 100% for the secondlaser beam of 1.32 micronmeters and a transmittivity greater than 90%for the second excitation laser beam of 0.807 micronmeter. The secondsemi-transparent mirror 32 may have a reflectivity of 98% for the secondlaser beam of 1.32 micronmeters.

Herein, it is to be noted that the third resonator 33a is tuned to athird level laser beam which has a wavelength of 0.94 micronmeter andwhich may be called the third laser beam 28. Specifically, the thirdreflection film has a reflectivity of about 100% for the third laserbeam of the wavelength of 0.94 micronmeter and a transmittivity greaterthan 90% for the excitation laser beam of 0.807 micronmeter. Such athird reflection film may be formed by a plurality of dielectric films,like the first and the second reflection films. In the third resonator33a, the third semi-transparent mirror 33 exhibits a reflectivity of 99%for the third laser beam of 0.94 micronmeter.

At any rate, the illustrated laser medium 21 is pumped from threedirections through each end surface. With this structure, the width L1,the length L2, and the thickness L3 of the laser medium 21 arepreferably as long as possible so as to effectively absorb theexcitation laser beams generated by the first through the thirdsemiconductor laser devices. On the other hand, they are preferably asshort as possible so as to reduce a loss in the laser medium 21 itself.Thus, two contradictory requirements should be fulfilled by the lasermedium 21. Taking this into consideration, optimum values of the widthL1, the length L2, and the thickness L3 are determined. In this event,consideration should be given to the fact that self-absorption in thelaser medium 21 rarely takes place in connection with the fourth levellaser beams, such as the first and the second laser beams of 1.32micronmeters and 1.06 micronmeters, while the self-absorption in thelaser medium 21 seriously takes place in connection with the third levellaser beam, such as the third laser beam having the wavelength of 0.94micronmeter.

It has been found that an optimum length of the laser medium for thethird level laser beam is less than an optimum length of the lasermedium for the fourth level laser beam. Accordingly, both the third andthe fourth level laser beams can not be emitted from the laser mediumwhich has a common length.

Taking the above into consideration, the illustrated laser medium 21 hasthe width L1 and the length L2 each of which is equal to 5 millimetersso as to oscillate the fourth level laser beams and the thickness L3equal to 1.5 millimeters so as to oscillate the third level laser beam.Thus, the third and the fourth level laser beams can be oscillated bythe use of a common laser medium 21 by making the thickness L3 differfrom the width L1 and the length L2.

As readily understood from the above, it is possible to emit the firstthrough the third laser beams 26 to 28 from the common laser medium 21along the A-axis, the B-axis, and the C-axis which are orthogonal to oneanother. Practically, it has been confirmed that the first through thethird laser beams 26 to 28 have the wavelengths of 1.06, 1.32, and 0.94micronmeters and output power between 100 and 150 milliwatts and aresimultaneously generated in the form of continuous waves. In the examplebeing illustrated, the first through the third laser beams 26 to 28 aregenerated as first through third output laser beams, respectively.

Referring to FIG. 3, a solid-state laser device according to a secondembodiment of this invention comprises similar parts designated by likereference numerals. In the illustrated example, the first through thethird laser beams 26 to 28 are emitted from the laser medium 21 alongthe A-axis, the B-axis, and the C-axis, respectively, and therefore havethe first through the third wavelengths of 1.06, 1.32, and 0.94micronmeters, respectively. First through third resonators depicted at31a', 32a', and 33a' are disposed so that they intersect the A-axis, andB-axis, and the C-axis, like in FIG. 2. The first through the thirdresonators 31a' to 33a' are somewhat different from those illustrated inFIG. 2, although the first through the third reflection films depositedon the laser medium 21 are the same as those illustrated in FIG. 2. Inaddition, second harmonic wave converters 41, 42, and 43 are insertedwithin the first through the third resonators 31a' to 33a' to divide thewavelengths of the first through the third laser beams in half and toproduce second harmonic wave beams 56, 57, and 58 of the first throughthe third laser beams, respectively. In this connection, the secondharmonic wave beams 56 to 58 of the first through the third laser beamshave wavelengths equal to 0.53, 0.66, and 0.47 micronmeters,respectively. To this end, the second harmonic wave converters 41, 42,and 43 are constructed in correspondence to the wavelengths of thesecond harmonic wave beams, as will be described later.

More specifically, the first resonator 31a' disposed along the A-axiscomprises a first semi-transparent mirror 31' which exhibits areflectivity of about 100% for the first wavelength of 1.06 micronmetersand a transmittivity of about 90% for the wavelength of 0.53micronmeter. The second resonator 32a' disposed along the B-axiscomprises a second semi-transparent mirror 32' which exhibits areflectivity of about 100% for the second wavelength of 1.32micronmeters and a transmittivity of about 90% for the wavelength of0.66 micronmeter. Likewise, the third resonator 33a' disposed along theC-axis comprises a third semi-transparent mirror which exhibits areflectivity of about 100% for the wavelength of 0.94 micronmeter and atransmittivity of about 90% for the wavelength of 0.47 micronmeter. Suchsemi-transparent mirrors may be readily formed by depositing a pluralityof dielectric films by the use of a known technique.

The second harmonic wave converter 41 between the laser medium 21 andthe first semi-transparent mirror 31' is formed by a crystal of KTP(KTiOPO₄) having a length of 5 millimeters along the A-axis while thesecond harmonic wave converter 42 between the laser medium 21 and thesecond semi-transparent mirror 32' is formed by another crystal of KTPhaving a length of 3 millimeters along the B-axis. In addition, thethird harmonic wave converter 43 is formed by a crystal of KNbO₃ havinga length of 4 millimeters along the C-axis.

With this structure, the first through the third laser beams which willbe called primary laser beams are confirmed within the first through thethird resonators 31a' to 33a' from which the second harmonic wave beams,such as 56 to 58, are derived. As a result, the first semi-transparentmirror 31a' passes through the second harmonic wave beam 56 of 0.53micronmeter which is a green-colored laser beam while the secondsemi-transparent mirror 32a' passes through the second harmonic wavebeam 57 of 0.66 micronmeter which is a red-colored laser beam. Likewise,the third semi-transparent mirror 33a' passes through the secondharmonic wave beam 58 of 0.47 micronmeter which is a blue-colored laserbeam. Such green-, red-, and blue-colored laser beams 56 to 58 areconcurrently generated with output power between 10 and 50 milliwattsand may be called first through third output laser beams. At any rate,three primary colored laser beams are obtained by the use of the singlelaser medium 21.

The three primary colored-laser beams are combined together into awhite-colored or external laser beam 60 by the use of first and secondreflective mirrors 61 and 62 and first and second combination mirrors 66and 67. In this event, the first reflective mirror 61 which is locatedin front of the second semi-transparent mirror 32' serves to totallyreflect the second harmonic wave beam of the wavelength of 0.66micronmeter in the direction of the second combination mirror 67 whilethe second reflective mirror 62 which is located upwards of the thirdsemi-transparent mirror 33' serves to totally reflect the secondharmonic wave beams of the wavelength of 0.47 micronmeter into the firstcombination mirror 66. In this connection, the first reflective mirror61 may reflect about 100% of this red-colored laser beam of 0.66micronmeter while the second reflective mirror 62 may reflect about 100%of the blue-colored laser beam of 0.47 micronmeter.

The first combination mirror 66 which intersects the A-axis in front ofthe first semi-transparent mirror 31' serves to reflect the blue-coloredbeam 0.47 micronmeter and to allow the green-colored laser beam of 0.53micronmeter to pass therethrough. On the other hand, the secondcombination mirror 67 which is positioned at the rear of the firstcombination mirror 66 serves to reflect the red-colored laser beam of0.66 micronmeter in the direction of the second combination mirror 67and to pass through both the laser beams which have the wavelengths of0.53 and 0.47 micronmeters.

With this structure, the blue-colored laser beam is reflected by thesecond reflective mirror 62 and further reflected along the A-axis bythe first combination mirror 66 which passes through the green-coloredlaser beam, as mentioned before. As a result, the blue-colored laserbeam is combined with the green-colored laser beam by the firstcombination mirror 66 into a combined laser beam. The combined laserbeam is sent to the second combination mirror 67 which is given thered-colored laser beam reflected by the first reflective mirror 61.Inasmuch as the second combination mirror 67 passes the combined laserbeam therethrough and reflects the red-colored laser beam, the combinedlaser beam and the red-colored laser beam are combined together into thewhite-colored laser beam 68 by the second combination mirror 67.

In the example being illustrated, it is possible to independentlycontrol output levels or intensities of the red-, the green-, and theblue-colored laser beams by adjusting output levels of the first throughthe third excitation laser beams. This means that it is possible toattain each color over the entire visible range. Accordingly, theillustrated laser device is operable as a tunable light source in thevisible range.

While this invention has thus far been described in conjunction with afew embodiments thereof, it will readily be possible for those skilledin the art to put this invention into practice in various other manners.For example, the laser medium 21 may be formed by a glass material orother materials except YAG. In this case, optical lengths in the lasermedium may be identical with those illustrated in the embodiments. Thelaser activators doped may be rare earth elements, such as erbium (Er),proseodymium (Pr), holmium (Ho), thulium (Tm). The laser medium 21 neednot always be a slab-shaped rectangular parallelpiped but a polyhedronwhich may be, for example, a hexagonal prism, an octagonal prism, or thelike. In this event, axes, such as the A-, the B-, and the C-axes, maynot be orthogonal to one another. When such a polyhedron is used as thelaser medium, first through N-th resonance optical paths may be formedthrough the laser medium which provides first through N-th internaloptical paths, respectively, when N is an integer greater than three. Inthis connection, first through N-th resonators should be preparedtogether with first through N-th excitation sources, such assemiconductor laser devices. Each of the second harmonic wave convertersmay be formed by a crystal of LiNbO₃, BNN (Ba₂ NaNb₅ O₁₅), mNA(meta-Nitro-Aniline), MNA (2 Metyl-4 Nitro-Aniline), or KDP.

What is claimed is:
 1. In a solid-state laser device operable inresponse to first through N-th excitation beams to generate firstthrough N-th output laser beams along first through N-th resonanceoptical paths, where N is representative of an integer which is not lessthan two, said solid-state laser device comprising a laser medium havingfirst through N-th internal optical paths which form parts of the firstthrough the N-th resonance optical paths, respectively, the improvementwherein the first through the N-th internal optical paths include atleast one of the first through the N-th internal optical paths having awavelength such that a third level laser beam is emitted through said atleast one of the first through the N-th internal optical paths and atleast one of the remaining internal optical paths having anotherwavelength such that a fourth level laser beam is emitted through atleast one of the remaining internal optical paths:said device furthercomprising: first through N-th resonator means defining said firstthrough N-th optical paths and supplied with the first through the N-thexcitation beams for pumping said laser medium to generate the firstthrougn the N-th output laser beams along said first through said N-thresonance optical paths, respectively, said first through said N-thoutput laser beams including the third and the fourth level laser beams.2. A solid-state laser device as claimed in claim 1, N being equal tothree, wherein said laser medium has a rectangular parallelpipedconfiguration defined by width, length, and thickness which determinethe first, the second, and the third internal optical paths,respectively, each of said width and said wavelength providing thelength for the fourth level laser beam while the thickness provides thewavelength for the third level laser beam.
 3. A solid-state laser deviceas claimed in claim 2, wherein said first resonator means comprises:afirst reflection film in contact with said laser medium for allowingsaid first excitation beam to pass therethrough and to reflect the firstoutput laser beam; a first semi-transparent mirror for partiallytransmitting the first output laser beam; said second resonator meanscomprising: a second reflection film in contact with said laser mediumfor allowing said second excitation beam to pass therethrough and toreflect the second output laser beam; a second semi-transparent mirrorfor partially transmitting the second output laser beam; said thirdresonator means comprising: a third reflection film in contact with saidlaser medium for allowing said third excitation beam to passtherethrough and to reflect the third output laser beam.
 4. Asolid-state laser device as claimed in claim 3, furthercomprising:combining means responsive to the first through the thirdoutput laser beams for combining the first through the third outputlaser beams into an external laser beam.
 5. A solid-state laser deviceas claimed in claim 3, further comprising:first wavelength convertingmeans between the laser medium and the first semi-transparent mirror forconverting the first output laser beam into a first converted laser beamhaving one half of the wavelength of the first output laser beam; secondwavelength converting means between the laser medium and the secondsemi-transparent mirror for converting the second output laser beam intoa second converted laser beam having one half of the wavelength of thesecond output laser beam; and third wavelength converting means betweenthe laser medium and the third semi-transparent mirror for convertingthe third output laser beam into a third converted laser beam having onehalf of the wavelength of the third output laser beam.
 6. A solid-statelaser device as claimed in claim 5, further comprising:combining meansresponsive to the first through the third converted laser beams forcombining the first through the third converted laser beams into anexternal laser beam.
 7. A solid-state laser device for generating aplurality of output laser beams along respective resonance opticalpaths, said solid-state laser device comprising a laser medium which hasa predetermined configuration to define a plurality of internal opticalpaths along said resonance optical paths, first means for defining afirst one of said resonance optical paths with a first predeterminedwavelength to generate a third level laser beam as one of said outputlaser beams, and second means for defining a second one of saidresonance optical paths with a second predetermined wavelength differentfrom said first predetermined wavelength to generate a fourth levellaser beam as one of the remaining output laser beams.
 8. A solid-statelaser device as claimed in claim 7 wherein said first means comprises asemi-transparent mirror on said one resonance optical path and filmmeans on said laser medium, said second means comprising a secondsemi-transparent mirror on the second resonance optical path and secondfilm means on said laser medium.
 9. A solid-state laser device asclaimed in claim 8 further comprising a resonating means on each opticalpath for producing an excitation laser beam along said path, eachresonating means being so located that the laser medium is disposedbetween the respective resonating means and an associated one of saidsemi-transparent mirrors.